Hourly Black Carbon concentrations usually scattered in the range of 0.5–2.0 mgm−3 in spring and autumn, and 0.0– 1.0 mgm−3 in fall and winter, corresponding with relatively high frequencies during these seasons. In contrast, the hourly Black Carbon distribution of background Waliguan site observed by Tang et al. [1999] showed a peak frequency in the range of 50–120 ng m−3, and a half‐cumulated frequency in 70–270 ng m−3. The highest frequency of hourly Black Carbon at urban Xi’an site fell into the range of 5–10 mgm−3[Zhu, 2008].